Method for producing polyester foam using a blowing agent combination

ABSTRACT

Low density PET foam is extruded by heating PET resin above the crystalline melt point to melt the resin, selecting a blowing agent combination, combining the blowing agent combination with the PET resin to create a mixture, cooling the mixture to a temperature of less than 538° K., and extruding the foam through a die. The blowing agent combination is characterized by about 50 to less than 100 mole percent of a first blowing agent having a boiling temperature at STP of greater than 310° K. The blowing agent combination is further characterized by more than 0 to about 50 mole percent of a second blowing agent having a boiling temperature at STP of less than 310° K. The blowing agent combination has an equilibrium solubility vapor pressure in PET of less than 45 atm at the foaming temperature and greater than or equal to 1 atm at the glass transition temperature.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention generally relates to a method for producingpolyester foam by extrusion. More particularly, the present inventionrelates to a method for producing low density polyester foam byextrusion.

2. Background Information

For some time, low density polystyrene foam has been found useful ininsulation, packaging, beverage cups and food containers. However,polystyrene foam and the extrusion process for making it have beenassociated with undesirable environmental concerns, regardless ofwhether those concerns have their origin in fact. In addition,polystyrene products generally have a service temperature limit of about366.3° K. Above the service temperature limit, the product will warp anddistort. Therefore, there is a general desire for other types of lowdensity foam that are not associated with such concerns.

Polyester resins, such as poly(ethylene terephthalate) (often referredto as "PET"), exist that could be used without such associated concerns.PET is currently widely used to make many recyclable plastic items, suchas soda bottles. However, attempts to produce a low density PET foamhave been less than entirely successful. The PET foams extruded usingsingle traditional blowing agents, such as, for example, carbon dioxide,chloro-difloro-methane and butane, have experienced foam cell collapseand/or severe corrugation. Thus, the quality of the PET foams producedhas not been close to polystyrene. The collapsing problem is due to thefact that the foaming temperature for PET is about 516° K., whichresults in a high rate of expansion for the traditional blowing agents,causing cell wall rupture and allowing the gas to escape. Without gas inthe PET foam cells prior to cooling, the cells cannot supportthemselves. In addition, PET resins generally have an inferior meltstrength compared to polystyrene resins. As one skilled in the art willknow, melt strength refers to the ability of a material to be stretchedat its melting temperature without breaking. The combination of a lowermelt strength and higher vapor pressure at the PET foaming temperaturealso requires a reduction in the size of the extrusion die opening wherethe foam exits. Such small die openings lead to a thin gauge foam sheetexperiencing severe corrugation at low densities.

Thus, a need exists for a way to make a quality low density polyesterfoam approaching or achieving the quality of polystyrene foam.

SUMMARY OF THE INVENTION

Briefly, the present invention satisfies the need for a qualitylow-density polyester foam by combining a high boiling point blowingagent providing plastization and volume with a low boiling point blowingagent providing the vapor pressure needed to prevent foam cell collapseduring cooling.

In accordance with the above, it is an object of the present inventionto provide a method for producing polyester foam by extrusion.

It is another object of the present invention to provide a method forproducing low-density polyester foam.

It is still another object of the present invention to provide a methodfor producing low-density, substantially uniform closed cell polyesterfoam.

It is yet another object of the present invention to provide a methodfor producing low-density, substantially uniform closed cell polyesterfoam with a surface that is substantially smooth to the touch.

The present invention provides a method for producing a substantiallyuniform closed cell foam of density less than 0.25 g/cm³ from apolyester resin by extrusion through a die at a foaming temperature. Themethod comprises steps of heating the polyester resin to the melttemperature (above 543° K.) to melt it, selecting a blowing agentcombination, combining the blowing agent combination with the polyesterresin to create a mixture, cooling the mixture to a temperature of lessthan 538° K., and extruding the foam from the die. The blowing agentcombination comprises about 50 to less than 100 mole percent of a firstblowing agent having a boiling temperature at STP of greater than 310°K., and more than 0 to about 50 mole percent of a second blowing agenthaving a boiling temperature at STP of less than 310° K. The blowingagent combination is characterized by an equilibrium solubility pressurein the polyester resin of less than about 45 atm at the foamingtemperature, and greater than or equal to 1 atm at a glass transitiontemperature.

These, and other objects, features and advantages of this invention willbecome apparent from the following detailed description of the variousaspects of the invention taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the major components of an extrusion systemuseful with the present invention.

FIG. 2 is a flow diagram for a general extrusion process with referenceto the system of FIG. 1.

FIG. 3 is a cross-sectional view of the extrusion die of FIG. 1.

FIG. 4 depicts a blown-up portion of the die of FIG. 3.

FIG. 5 is a graph of vapor pressure versus blowing agent type fortypical polystyrene foaming temperatures.

FIG. 6 is similar to FIG. 5, but at typical polyester foamingtemperatures.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to FIGS. 1-3, a general tandem extrusion process will nowbe described that is useful with the present invention. It will beunderstood, however, that other extrusion processes exist that couldalso be used, and this is merely one example given in order to put theinvention in context.

FIG. 1 is a block diagram of the major portions 10 of machinery used ina tandem extrusion process. The major portions 10 include a primaryextruder 12, secondary extruder 14 and die 16. One of ordinary skill inthe art will understand the operation of the major portions. Generally,heating of the solids 13 to be extruded (a polymer) and mixing with theblowing agent 15 are accomplished in primary extruder 12. Cooling of themixture is performed in secondary extruder 14. Finally, the cooledmixture is fed to die 16 for foaming.

FIG. 2 is a flow diagram for the extrusion process of FIG. 1. The rawmaterials are first fed to primary extruder 12 (STEP 20, "FEED RAWMATERIALS"). The raw materials will generally comprise a mixture ofvirgin polymer, reclaim polymer generated in manufacturing, colorants,stabilizers, nucleators, flame retardants, placticisers, and possiblyother additives. Although ratios of the additives may vary greatly,generally the virgin polymer and reclaim polymer constitute about 90% ormore of the solid feed by weight. The raw materials may be fed toprimary extruder 12 by volumetric or gravimetric feeders and may or maynot use a blender to homogenize the mixture before being fed. Often, theprimary extruder is flood fed; that is, there is a constant supply ofraw material directly on the extruder inlet or feed throat, althoughother types of feeding are practiced.

After the raw materials are fed to primary extruder 12, they arecompressed and heated to melt them (STEP 22, "COMPRESS AND HEAT"). Aftermelting the raw materials, the melt is pressurized (STEP 24, "PRESSURIZEMELT"). Typical pressures range from about 150 atm to about 350 atm.After pressurizing the melt, a blowing agent or agents (e.g.,hydrocarbons, halohydrocarbons and/or inert gases) is injected intoprimary extruder 12. The pressure may temporarily be reduced to aid inthe injection. The melted raw materials and blowing agent are then mixedto create a homogeneous mixture prior to exiting primary extruder 12(STEP 26, "MIX WITH BLOWING AGENT"). The mixing could be, for example,distributive or dispersive.

After injecting the blowing agent and combining with the melted rawmaterials, the mixture is generally too hot to foam. When the mixture istoo hot, viscosity is low, and if foaming were attempted, the blowingagent would expand the cells within the foam too rapidly, leading tocell wall rupture and foam collapse. If, on the other hand, the mixturewere too cold, the blowing agent would have insufficient potentialenergy to expand the mixture into a foam. Precise control of the foamingtemperature is thus needed to ensure good quality foam.

Cooling of the mixture is accomplished in secondary extruder 14 (STEP28, "COOL MIXTURE"). The secondary extruder is usually larger than theprimary extruder to maximize the amount of surface area for heattransfer. Shear heating of the mixture is minimized through variousdesigns for the secondary extruder screw, which provides continuoussurface renewal. Without this renewal, the mixture at the surface of theextruder barrel would freeze and insulate the rest of the mass, whichwould pass through the secondary extruder without being cooled. Usually,the extruder barrel in the secondary extruder operates at much lowerrevolutions than that of the primary extruder, to reduce shear heating.The particular screw design used may affect the pressure of the mixture.

The cooled mixture is then delivered to die 16 for foaming (STEP 30,"FOAM MIXTURE"). The principle purpose of the die is to shape thepolymer into a form, while maintaining the pressure to ensure that theblowing agent does not separate from the mixture prematurely. Ideally,the blowing agent remains in the mixture until exiting the die. Thedesign of the die determines the shape/thickness of the extruded foam.After the foam is extruded, any number of finishing equipmenttechnologies may be used to produce the final product.

FIG. 3 is a cross-sectional view of an exemplary embodiment of die 16.The cooled mixture enters die 16 through openings 32 and 34. Thetemperature and pressure of the mixture while inside the body of die 16are monitored by instrumentation (not shown) connected at taps 36 and38. The female die lip 40 and male die lip 42 define a die gap 43 (shownmore clearly in FIG. 4. Spider or breaker plate section 44 holds maledie lip 42 in place. Ideally, foaming takes place upon the polymermixture exiting through die gap 40 and the blowing agent thenvaporizing. However, foaming may begin in the land area 46 (called"prefoaming"). When prefoaming occurs, the result often is a lessdesirable surface finish and an irregular cell structure. To affect diepressure, the die gap may be increased or decreased through variousmechanisms. One example of such a mechanisms is screw 47 in FIG. 3.

Producing good quality polystyrene foam by thermal extrusion requiresthat the blowing agent used remain in solution with the polystyreneresin upon entering the die land. As used herein, the term "good qualityfoam" refers to a substantially uniform closed cell foam with a surfacethat is substantially smooth to the touch. If the blowing agent comesout of solution prior to entering the land, pre-foaming occurs,resulting in a poor surface, nonuniform cells and a weak foam.Acceptable quality polystyrene foam is produced if the blowing agentcomes out of solution after entering the land. However, good qualityfoam is produced if the blowing agent remains in solution until exitingthe land. When this condition is satisfied, a clear "halo" ofpolystyrene can seen external to the die before foam growth has causedthe material to become opaque. An operator of the extrusion machinerycan use this halo as a visual indication that the die pressure isadequate and a good quality foam is most probably being produced.

The following variable relationships have been found useful indetermining the conditions necessary to produce good quality polystyrenefoam for a given blowing agent: ##EQU1## where α means "proportionalto", * means "multiplied by", P_(D) is the actual pressure at the die inatmospheres, R is the foam production rate in kg/hour, BAC is theblowing agent concentration expressed as a percentage of total solutionweight, TM is the foaming temperature in °C., GAP is the die gap in mm,DR is the die radius in cm, and P_(ES) is blowing agent equilibriumsolubility pressure in psi. Typically, P_(D) is greater than P_(ES) byabout 20 to 40 atm to produce good quality foam. Additional pressure mayimprove the surface quality and foam cell structure, but at the cost ofreducing throughput. Although the viscosity of the polymer resin andtype of additives (e.g., talc to improve nucleation) also affect diepressure, variables other than those in relationships (1) and (2) aboveare assumed to be fixed by product requirement (e.g., the need for aflame retardant), or can be compensated for. As an example, temperaturecan be varied to compensate for viscosity.

Several additional assumptions greatly simplify relationship (1) above.The production rate is generally maximized for a given production line.The die radius is determined by the desired blow-up ratio (i.e., theratio of the circumference of the die to the final width of the extrudedfoam sheet). Generally, this ratio will result in the smallest diediameter that will still allow the extruded foam to be easily handledwithout an undue amount of corrugation. As one skilled in the art willknow, the term "corrugation" describes foam extruded with alternatingthick and thin areas, where the surface of the foam sheet can becharacterized by a sine wave relationship. Finally, the lowestachievable melt temperature is generally used for a given productionrate. Thus, based on a given production geometry, these variables become"fixed".

Based on these additional assumptions, relationship (1) reduces to:##EQU2##

The GAP varies, depending on the desired sheet thickness. Generally, aGAP of less than about 0.4 mm is undesirable, as dirt is more likely toinhibit extrusion and a foam thickness of greater than 10 times the GAPis generally difficult to achieve. Thus, 0.4 mm will be treated as thepractical minimum die GAP. In addition, the BAC is determined by thedesired density, as described more fully below. Therefore, for practicalpurposes, no degrees of freedom remain for a given blowing agent, andthe blowing agent selected will either fall within a practical operatingwindow for relationship (3) or it will not. Such a window 52 is shown ingraph 50 of FIG. 5 for polystyrene at 373° K. and 411° K. for a typicaldensity of 0.1 g/cm³.

Materials that meet the above parameters for use as a blowing agent forpolystyrene include butanes, pentanes and hexanes. Although othermaterials also exist that could be used as blowing agents according tothe above requirements, notably hydro-halocarbons, only alkanes will bediscussed here to illustrate the point. As can be seen in FIG. 5, vaporpressures above approximately 45 atmospheres result in corrugation ofthe product yielding irregular strength and inferior appearance. Propaneis expected to and does in fact result in product corrugation. A smallamount of corrugation (about 10% thickness variation or less) is seenwhen using n-Butane. However, this corrugation can be "formed out" ofthe product during thermoforming operations. Pressures corresponding tothis slight amount of corrugation range form 25 to 45 atmospheres. Atthe other end of graph 50 are blowing agents that will not support thedensity of the product as the polystyrene mass cools and "freezes" atits glass transition temperature of approximately 373° K. As theinternal cell gas pressure of n-Octane is less than 1 atmosphere at theglass transition temperature, collapse will occur. This is also true ofn-Heptane to a degree, since equilibrium vapor pressure is somewhat lessthan ideal vapor pressure. (Some amount of the blowing agent is insolution with the polymer, thereby lowering the overall vapor pressure.)Theory and experiment agree as some amount of collapse is seen withn-Heptane. It can therefore be seen that n-Pentane represents a nearprefect blowing agent for polystyrene using conventional foamingtechnology. No corrugation is expected and no collapse of the product isexpected as the product cools. This is in fact what is seen in the realworld, and n-Pentane is widely used as a blowing agent.

In the recent past, PET manufacturers have dedicated substantialresources to producing modified PET resins with melt strengthsapproaching that of polystyrene. Thus, it is a reasonable starting pointto assume that relationships (1) and (2) above for polystyrene foamblowing agents will hold true for PET foam blowing agents as well.

As with polystyrene, the PET foam production rate is determined by theextrusion equipment used, power requirements, etc. Also, the blowingagent concentration is again set according to the desired density, andthe die gap is set according to the desired PET foam thickness. The dieblow-up ratio for PET foam is similar to, but slightly smaller than thatfor polystyrene, since PET foam has proven difficult to handle atblow-up ratios of over three-to-one. As one skilled in the art willknow, the "blow-up ratio" refers to the circumference of the mandraldetermining the final width of the sheet compared to the circumferenceof the die. Polystyrene foam is generally produced at up to afour-to-one blow-up ratio at typical commercial densities. Finally, themelt temperature for PET is significantly different than that forpolystyrene. Polystyrene is foamed at about 411° K., whereas PET isfoamed at about 516° K. At this higher temperature, traditional blowingagents will cause severe corrugation and/or cell collapse, due to anincrease in vapor pressure.

Corrugation results when the growth rate of the foam exceeds thephysical geometric constraints of the system. For a given product, it isdesired to obtain a given density and thickness. The density iscontrolled through the moles of gas added and the thickness iscontrolled through the pull or take-away speed. When producing foam,bubble formation initiates a three-dimensional growth. Since thicknessis controlled and machine direction speed limited by rate, the onlyremaining dimension for growth is radial. In the manufacture of foamsheet, as indicated above, a blow-up ratio of 3 to 4 is typical. Thesheet exits the die with a finite velocity and transitions from the diediameter to the mandrel diameter at a maximum angle of 90 degrees (45 to70 degrees is common). If the rate of bubble growth increases the sheetcircumference more quickly than the blow-up geometry allows, corrugationwill occur. Corrugation can be described as a sine wave pattern duringgrowth about a circular centerline. The result of this corrugation isthat permanent lines are produced in the sheet in the machine directioncorresponding to areas that are relatively thicker and thinner next toeach other. The thinner areas when formed into a final product yieldweak bands in the product. The result is poor product strength, heavierpart weight to offset the poor strength, and poor appearance.

There are three major factors that determine corrugation: density,volatility of the blowing agent, and cell size. As the desired densitydecreases, the expansion ratio (density of the polymer melt divided bythe density of the final foam) increases. Generally, product thicknessrequirements are set and the resulting increase in volumetric growthmust be accommodated in the machine and radial directions. If the foamis to retain a relatively balanced cell structure, both must beincreased by a power of 1.73 (i.e., the square root of 3). That is,since:

    expansion ratio=k×(radial growth)×(thickness growth)×(machine direction growth),

where k is a constant, if the expansion ratio is increased, butthickness is fixed, the equation becomes:

    expansion ratio=k×(radial growth).sup.1.73 ×(machine direction growth).sup.1.73.

The exponents can be manipulated somewhat to attempt to reduce theradial growth, but it can be clearly seen that a reduction in densityresults in a higher radial growth rate, and, therefore, a greaterpossibility of corrugation.

The volatility of the blowing agent affects corrugation in thatincreased volatility results in more rapid cell growth. This often meansthat the final density of the product is reached or nearly reachedbefore the product achieves the diameter of the mandrel, and, therefore,corrugation results. Low volatility blowing agents result in slow bubblegrowth and, therefore, no corrugation, but these gases may not containthe necessary potential energy to expand the foam to the desired densityor may even become liquid during the expansion process, resulting inproduct collapse.

The cell size affects corrugation in the following way. The bubblegrowth rate is determined by the ability of the blowing agent topermeate to a nearby cell site and become gaseous. If more nucleant isused and the cells are smaller, the mean distance for the blowing agentto permeate to a cell is reduced. The result is faster cell growth and,again, a greater possibility of corrugation. It is theorized that as thecell size approaches zero and the number of cells approaches infinity,the growth rate of the foam approaches infinity, and the product willcorrugate with any geometry.

Blowing agent combinations, according to the present invention, reducecorrugation. No selection of gases can affect the desired blow-up ratio;that is, the blow-up ratio is an independent variable. Cell size is alsoa controlled variable chosen for reasons of product strength,flexibility, or appearance. The proper blend of blowing agents can,however, reduce the overall volatility of the system. The highervolatility component will expand first. Since the lower volatilitycomponent contributes little to bubble growth in this early phase, theeffect is that the relative expansion ratio is low, and, therefore, theopportunity for corrugation is greatly reduced. The lower volatilityblowing agent then contributes a slower bubble growth, achieving thedesired density without corrugation. During this phase, the partialpressure of the higher volatility blowing agent in the cells helps toprevent collapse of the foam structure.

As used herein, corrugation expressed as a percentage refers to thepercentage difference between the maximum sheet thickness and minimumsheet thickness in adjoining bands or strips. In other words, it is thepercent deviation about a local mean thickness. A level of up to 10% incorrugation can usually be tolerated, as the forming operation oftenwill ultimately still yield a fairly uniform product.

Since variables for PET other than melt temperature are similar to thatfor polystyrene, the blowing agents useful for PET should be those thatexhibit vapor pressures at the PET foaming temperature similar to thatexhibited by the traditional blowing agents at the polystyrene foamingtemperature. Based on this simple premise, merely shifting the practicaloperating area in graph 50 of FIG. 5 to the right should result inblowing agent choices providing good quality PET foam. FIG. 6 graphsn-Pentane through n-Octane, similar to FIG. 5, at temperatures of 516°K. and 343° K. (the melt temperature at the die and glass transitiontemperature for PET, respectively) for a typical density of 0.1 g/cm³.Note that the vapor pressures used to construct the graphs of FIGS. 5and 6 were estimated using Antoine Equations as presented in TheProperties of Gases & Liquids, 3d Ed., by Reid, Penusnitz and Sherwood,McGraw Hill, 1977, p. 629-665, which is hereby incorporated byreference.

Examination of FIG. 6 reveals, however, that simply shifting the graphtoward higher molecular weight blowing agents is ineffective. Unlikepolystyrene, there is a large temperature difference between the foamingtemperature, which must be above the crystalline melt temperature, andthe glass transition temperature. (To form product, the extruded sheetis quenched using a chilled roll or mandrel. This results in anamorphous sheet, but also allows for molecular movement down to theglass transition temperature of about 343° K.) Graph 54 suggests thatn-Pentane will cause excessive corrugation, while some collapse of thefoam will occur when using n-Heptane. Both of these suggestions havebeen experimentally verified. From FIG. 6, it can be seen that n-Hexanesatisfies the selection criteria previously discussed. Although throughcareful control of operating parameters, n-Hexane can be usedsuccessfully to produce low density PET foam, its toxicity makes itunsuitable for many commercial uses.

To consistently produce "good quality" PET foam, it is clear that few,if any, single blowing agents can be used. However, by blending two ormore blowing agents, the desired characteristics can be obtained. Forexample, a two-parts n-Octane to one-part n-Pentane would have thefollowing properties. Rault's Gas Law provides that partial pressuresare additive, therefore:

    pressure (at foaming)=(0.67×11.22 atm)+(0.33×54.63 atm)=25.5 atm,

thus meeting the desired maximum vapor pressure needed to eliminate ornearly eliminate corrugation. (As stated before, a small amount ofcorrugation can be compensated for in the forming operation.) To preventcollapse:

    pressure (at glass transition temperature)=(0.67×0.14 atm)+(0.33×2.79 atm)=1.01 atm,

thus meeting the desired minimum vapor pressure needed to preventcollapse. Clearly then, there are many possible combinations of two ormore blowing agents that will yield good quality foam.

The present invention provides a method for extruding good quality, lowdensity polyester foam. The term "low-density" used in this contextrefers to an extruded polyester foam of a density less than about 0.25g/cm³. A polyester resin is first heated to the foaming temperature,which depends on the resin used, but is above its crystalline melt point(about 543° K.). A combination of blowing agents is selected forcombining with the polyester resin.

As used herein, the term "polyester resin" refers to a polyester resinwith an intrinsic viscosity of more than about 0.8 dl/g.

The blowing agent combination comprises about 50 to less than 100 molepercent of a first blowing agent having a boiling temperature at STP(Standard Temperature and Pressure) of greater than 310° K. Examples ofblowing agents that could be used as the first blowing agent includeheptane and octane. The first blowing agent, having a relatively highboiling temperature, provides plastization and volume for the foam. Theblowing agent combination also comprises more than 0 to about 50 molepercent of a second blowing agent having a boiling temperature at STP ofless than 310° K. Examples of blowing agents that could be used as thesecond blowing agent include butane, tetrafloroethane, carbon dioxideand pentane. The second blowing agent, having a relatively low boilingtemperature, provides the vapor pressure to prevent collapse at theglass transition temperature. In addition, high vapor pressurerepresents potential energy that, upon exiting the die, is converted towork used to expand the foam against the visco-elastic resistance of thepolymer. This high vapor pressure component is useful in achieving lowdensities. The blowing agent combination is characterized as having anequilibrium solubility vapor pressure in PET of less than 45 atm at thefoaming temperature and greater than or equal to 1 atm at the glasstransition temperature.

After the blowing agent combination is selected, it is combined with thepolyester resin to create a mixture. The mixture is then cooled to atemperature of less than 538° K., and preferably less than 528° K. Thecooled mixture is then extruded through the die. For good quality foam,the blowing agent combination should stay in solution with the resinuntil at least until entering the die land, and preferably until exitingthe die land.

Example data will now be presented. All of the examples use the samepolyester resin. However, examples 1-4 use a single blowing agent, whileexamples 5 and 6 use a combination of different blowing agents. Inaddition, typical nucleators are used in each example.

EXAMPLE 1

This example (see Table I below) illustrates one of the two majorproblems associated with conventional blowing agents. At a level of 1%iso-pentane, which yields a relatively high density of 0.23 g/cm³,unacceptable levels of corrugation were produced. Compared to theaverage thickness of 1.08 mm, the thickness of the sheet at the high andlow points varied plus and minus 20%. This variation in thickness orcorrugation caused weak bands to be formed into the product and resultedin an unacceptable product quality. (Although somewhat arbitrary,corrugation levels of up to 10% are generally considered acceptable. Theforming operation can successfully remove such a level of corrugationwithout deleterious effects to the final product.) Note that at thisdensity, the efficiency of the blowing agent is quite high. The measuredfoam density was 96% of the theoretical density attainable, based on themass of material extruded and the amount of blowing agent added.

The theoretical density was calculated based on the ideal gas law. Themass of the solid additives was converted to a volume based uponspecific gravity. This amount of volume was added to the gaseous volumeand divided into the extruded mass to yield a density. To determine thegaseous volume, the ideal gas law was used. The blowing agent weightadded was converted to moles. By the ideal gas law, each mole yields22.4 liters of volume at 1 atm and 298° K. The gas in the expanding masscan be assumed to expand until internal and external pressures reachequilibrium. The polyester only expands until reaching the glasstransition temperature of about 343° K., at which time the foam became"frozen" and no further movement of the polyester molecules occurred.Using the elevated temperature of 343° K., a further reduction of 15% inthe density is attainable due to the additional expansion of a gaseousmole to 25.8 liters. Again, this volume is based on the ideal gas law.Assumptions in this model include: the ideal gas law is applicable (thisis a good assumption as pressures were low and the gases were notcritical), and the residual solubility of the blowing agent in thepolymer at the glass transition temperature is small (based ontemperature and pressure solubility data, this is also considered avalid assumption).

                  TABLE I                                                         ______________________________________                                        Temperatures                                                                  2.5" Cooling Extruder 254° C.                                          Die Temperature       267° C.                                          Die Lip               253° C.                                          Crossover Melt        282° C.                                          Die Melt              254° C.                                          Pressures                                                                     Injection Pressure    183.7 atm                                               2" Pressure           233.3 atm                                               Crossover Pressure    191.2 atm                                               Die Pressure          41.5 atm                                                Drive Conditions                                                              2" AMP                34 amps                                                 2" Speed              70 rpm                                                  2.5" AMP              14 amps                                                 2.5" Speed            16.5 rpm                                                Formulation                                                                   Polymer Rate          29.2 kg/hr                                              Nucleator Rate        0.36 kg/hr                                              Blowing Agent Type    i-Pentane                                               Blowing Agent Rate    0.30 kg/hr                                              Test Data                                                                     Density               0.23 g/cm.sup.3                                         Theoretical Density   0.22 g/cm.sup.3                                         Average Thickness     1.08 mm                                                 Corrugation           20%                                                     Cell Size             0.18 mm                                                 ______________________________________                                    

EXAMPLE 2

This example (see Table II below) illustrates the problem of collapseassociated with conventional blowing agents used in an effort to makepolyester foam. In this case, the amount of gas was increased to a levelyielding a theoretical density of 0.11 g/cm³. Due to the high vaporpressure of the gas at the foaming temperature, the growth rate of thefoam exceeded the melt strength of the polymer. The result was open cellfoam and complete collapse.

                  TABLE II                                                        ______________________________________                                        Temperatures                                                                  2.5" Cooling Extruder 252° C.                                          Die Temperature       258° C.                                          Die Lip               259° C.                                          Crossover Melt        276° C.                                          Die Melt              252° C.                                          Pressures                                                                     Injection Pressure    170.1 atm                                               2" Pressure           195.9 atm                                               Crossover Pressure    174.2 atm                                               Die Pressure          48.3 atm                                                Drive Conditions                                                              2" AMP                35 amps                                                 2" Speed              80 rpm                                                  2.5" AMP              10 amps                                                 2.5" Speed            17.0 rpm                                                Formulation                                                                   Polymer Rate          33.4 kg/hr                                              Nucleator Rate        0.24 kg/hr                                              Blowing Agent Type    i-Pentane                                               Blowing Agent Rate    0.76 kg/hr                                              Test Data             >1.0 g/cm.sup.3                                         Density               0.11 g/cm.sup.3                                         Theoretical Density   NA                                                      Average Thickness     NA                                                      Corrugation           NA                                                      Cell Size             NA                                                      ______________________________________                                    

EXAMPLE 3

This example (see Table III below) illustrates the problem associatedwith high boiling point gases that have insufficient vapor pressure atthe polyester glass transition temperature to support the foamstructure. In this example, a level of 1.4% n-Heptane was used as theblowing agent yielding on a mole basis the same theoretical density asExample 1. The actual measured density is, however, 0.29 g/cm³, whichresults in a blowing agent efficiency of only 76% versus 96% inExample 1. Since the vapor pressure of n-Heptane is insufficient to keepthe cell fully expanded as the foam cools, some level of collapse wasobserved, which accounts for the low blowing agent efficiency. Inaddition, a cell that has collapsed to any degree will have a wrinkledcell wall that is much weaker than a cell of equal density with rigidcell walls. There is, therefore, a double penalty for collapse: higherdensity resulting in higher part weight; and weaker structure resultingin weaker product or still higher part weight. Note that the level ofcorrugation measured was less than 10%.

                  TABLE III                                                       ______________________________________                                        Temperatures                                                                  2.5" Cooling Extruder 254° C.                                          Die Temperature       263° C.                                          Die Lip               257° C.                                          Crossover Melt        282° C.                                          Die Melt              254° C.                                          Pressures                                                                     Injection Pressure    190.5 atm                                               2" Pressure           240.8 atm                                               Crossover Pressure    195.2 atm                                               Die Pressure          59.9 atm                                                Drive Conditions                                                              2" AMP                36 amps                                                 2" Speed              70 rpm                                                  2.5" AMP              14 amps                                                 2.5" Speed            18.5 rpm                                                Formulation                                                                   Polymer Rate          29.2 kg/hr                                              Nucleator Rate        0.36 kg/hr                                              Blowing Agent Type    n-Heptane                                               Blowing Agent Rate    0.42 kg/hr                                              Test Data                                                                     Density               0.29 g/cm.sup.3                                         Theoretical Density   0.22 g/cm.sup.3                                         Average Thickness     0.97 mm                                                 Corrugation           6%                                                      Cell Size             0.17 mm                                                 ______________________________________                                    

EXAMPLE 4

In this example (see table IV below), the level of n-Heptane wasincreased to 1.8% in an effort to produce a sample with a density ofless than 0.25 g/cm³. The theoretically attainable density for thissample is 0.18 g/cm³. The result of increasing the gas level was,however, to produce a higher density product than Example 3. The densityof the product at some point during cooling did approach the theoreticalvalue, however, that resulted in thinner cell walls that were even lesscapable of resisting cell collapse than Example 3. The sheet producedhad a density of 0.37 g/cm³ and a blowing agent efficiency of only 49%.Note that although the level of corrugation expected was relatively low,the actual measured value of 15% was believed to be due to preferentialcollapse along lines of otherwise minor levels of corrugation.

                  TABLE IV                                                        ______________________________________                                        Temperatures                                                                  2.5" Cooling Extruder 255° C.                                          Die Temperature       267° C.                                          Die Lip               259° C.                                          Crossover Melt        282° C.                                          Die Melt              255° C.                                          Pressures                                                                     Injection Pressure    163.3 atm                                               2" Pressure           231.3 atm                                               Crossover Pressure    163.3 atm                                               Die Pressure          53.7 atm                                                Drive Conditions                                                              2" AMP                35 amps                                                 2" Speed              70 rpm                                                  2.5" AMP              14 amps                                                 2.5" Speed            20.5 rpm                                                Formulation                                                                   Polymer Rate          28.8 kg/hr                                              Nucleator RaLe        0.72 kg/hr                                              Blowing Agent Type    n-Heptane                                               Blowing Agent Rate    0.54 kg/hr                                              Test Data                                                                     Density               0.37 g/cm.sup.3                                         Theoretical Density   0.18 g/cm.sup.3                                         Average Thickness     0.83 mm                                                 Corrugation           15%                                                     Cell Size             0.13 mm                                                 ______________________________________                                    

EXAMPLE 5

This example (see Table V below) illustrates the present invention. Theblowing agent used is a blend of 55% by weight n-Heptane with 45% byweight iso-pentane. Both of these blowing agents have been shown byexample to produce inferior product by themselves. Combined in thisratio, the calculated vapor pressure of the blend (which is by Roult'slaw additive) was 36 atm at 516° K. and 1.80 atm at 343° K., satisfyingthe conditions for selection as a successful blowing agent combination.The resulting example supports the selection criteria. The sampleproduced had a density of 0.23 g/cm³, a blowing agent efficiency of 84%,and corrugation of only 3%.

                  TABLE V                                                         ______________________________________                                        Temperatures                                                                  2.5" Coolig Extruder  254° C.                                          Die Temperature       268.5° C.                                        Die Lip               258° C.                                          Crossover Melt        283° C.                                          Die Melt              254° C.                                          Pressures                                                                     Injection Pressure    217.7 atm                                               2" Pressure           212.9 atm                                               Crossover Pressure    217.0 atm                                               Die Pressure          83.7 atm                                                Drive Conditions                                                              2" AMP                34 amps                                                 2" Speed              85 rpm                                                  2.5" AMP              12 amps                                                 2.5" Speed            17.5 rpm                                                Formulation                                                                   Polymer Rate          33.4 kg/hr                                              Nucleator Rate        0.24 kg/hr                                              Blowing Agent Type    Blend 1*                                                Blowing Agent Rate    0.54 kg/hr                                              Test Data                                                                     Density               0.23 g/cm.sup.3                                         Theoretical Density   0.18 g/cm.sup.3                                         Average Thickness     0.99 mm                                                 Corrugation           3%                                                      Cell Size             0.20 mm                                                 ______________________________________                                         *55% nHeptane/45% iPentane                                               

EXAMPLE 6

This example (see Table VI below) further illustrates the presentinvention. The blowing agent used is a blend of 70% by weight c-Pentanewith 30% by weight iso-Pentane. When combined in this ratio, a densityof 0.19 g/cm³ was achieved resulting in a blowing agent efficiency of100% (within experimental error). The measured level of corrugation forthis sample was 5%.

                  TABLE VI                                                        ______________________________________                                        Temperatures                                                                  2.5" Cooling Extruder 252° C.                                          Die Temperature       268° C.                                          Die Lip               261° C.                                          Crossover Melt        277° C.                                          Die Melt              252° C.                                          Pressures                                                                     Injection Pressure    210.9 atm                                               2" Pressure           212.9 atm                                               Crossover Pressure    215.0 atm                                               Die Pressure          59.2 atm                                                Drive Conditions                                                              2 " AMP               35 amps                                                 2" Speed              80 rpm                                                  2.5" AMP              16 amps                                                 2.5" Speed            18.0 rpm                                                Formulation                                                                   Polymer Rate          36.0 kg/hr                                              Nucleator Rate        0.36 kg/hr                                              Blowing Agent Type    Blend 2*                                                Blowing Agent Rate    0.42 kg/hr                                              Test Data                                                                     Density               0.19 g/cm.sup.3                                         Theoretical Density   0.19 g/cm.sup.3                                         Average Thickness     1.22 mm                                                 Corrugation           5%                                                      Cell Size             0.22 mm                                                 ______________________________________                                         *30% iPentane/70% cPentane                                               

While several aspects of the present invention have been described anddepicted herein, alternative aspects may be effected by those skilled inthe art to accomplish the same objectives. Accordingly, it is intendedby the appended claims to cover all such alternative aspects as fallwithin the true spirit and scope of the invention.

I claim:
 1. A method for producing a substantially uniform closed cellfoam of density less than about 0.25 g/cm³ from a polyester resin withan intrinsic viscosity of more than about 0.8 dl/g by extrusion througha die at a foaming temperature, comprising steps of:heating thepolyester resin to the foaming temperature such that the polyester resinmelts, wherein the melt temperature is above 543° K.; selecting ablowing agent combination, comprising: about 50 to less than 100 molepercent of a first blowing agent having a boiling temperature at STP ofgreater than 310° K.; and more than 0 to about 50 mole percent of asecond blowing agent having a boiling temperature at STP of less than310° K.; wherein the blowing agent combination has an equilibriumsolubility pressure in the polyester resin of less than about 45atmospheres (atm) at the foaming temperature and greater than or equalto 1 atm at a glass transition temperature; combining the blowing agentcombination with the polyester resin to create a mixture; cooling themixture to a temperature of less than 538° K.; and extruding thesubstantially uniform closed cell foam of density less than about 0.25g/cm³ from the die; wherein the polyester resin comprises poly(ethyleneterephthalate), wherein the first blowing agent comprises heptane,octane, or cyclopentane, and wherein the second blowing agent comprisesbutane, tetrafloroethane, carbon dioxide, i-pentane or n-pentane.
 2. Themethod of claim 1 wherein the blowing agent combination is unreactablewith the polyester resin.
 3. The method of claim 1 wherein the densityof the substantially uniform closed cell foam is in a range from 0.02g/cm³ to less than 0.25 g/cm³.
 4. The method of claim 1 wherein theblowing agent combination is non-toxic.
 5. The method of claim 1 whereinthe polyester resin comprises one or more additives.
 6. The method ofclaim 5 wherein the one or more additives comprise one or more of a cellsize nucleation agent, a flame retardant, a colorant and a plasticizer.7. The method of claim 1 wherein the substantially uniform closed cellfoam has corrugation equal to or less than 10%.